a) Field of the Invention
This invention relates to an X-ray microscope favorable to microscopy for biological specimens and the like.
b) Description of the Prior Art
Recently, for the purpose of microscopy for biological specimens, the development of a microscope has taken place which is suitable for a soft X-ray region, and more particularly for X rays with wavelengths of nearly 20-44 .ANG. termed "water window".
This microscope is intended to secure an image of an object by radiating the X rays emitted from a radiation source onto the object to detect radiation transmitted through the object and secondary radiation generated from the object by an X-ray detector.
For the source of soft X rays, an electron-beam source bringing about characteristic X rays is commercially available. In addition to this, a synchrotron very excellent in brightness and a laser plasma source excellent in brightness and repetition performance have come into wide use for research.
FIG. 1A shows the spectrum of light emitted from a synchrotron radiation source [G. Schmahl, "X-Ray Microscopy", Springer Series in Optical Sciences, Vol. 43, (Springer-Verlag) (1983)] and it will be seen from this diagram that synchrotron radiation is white light with a wide wavelength band. Further, FIG. 1B depicts the spectrum per unit solid angle and unit laser pulse of light emitted from a typical laser plasma source [Kazuo Tanaka, "Optical and Electro-optical Engineering Contact", Japan Optoelectro-Mechanics Association, Vol. 27, No. 4, p. 187 (1989)] and it will likewise be seen from this diagram that the laser plasma source also emits the white light, although its bandwidth is narrow compared with that of radiation light.
On the other hand, as optical systems in which the radiation coming from an X-ray source is converged onto the object and the radiation from the object is converged onto the detector are known, for example, reflecting optical systems like a Walter optical system such as is shown in FIG. 2A (in which a ray of light produced by reflecting mirrors assuming an ellipsoid and a hyperboloid formation is made incident at a grazing angle smaller than a critical angle) and a Schwarzschild optical system such as is shown in FIG. 2B and a diffracting optical system making use of a zone plate such as is shown in FIG. 2C.
Further, as detectors indicating the radiation of a wavelength region ranging from the X rays to vacuum ultraviolet rays are known an MCP (microchannel plate), channeltron, CCD image sensor, imaging plate, X-ray film, photoresist, etc.
The MCP is an electron multiplier with a high grade of efficiency which, in general, is widely used for the detection of charged particles and radiation. A typical MCP assumes such geometry as is shown in FIG. 3A. Its channels, which in most cases, are each about 10-50 .mu.m in diameter, are electrically connected in parallel by electrodes located at the front and rear faces of the MCP, as shown in FIG. 3B, which are supplied with large bias voltages. When the charged particles and light are radiated, electrons are produced in the microchannel by a photoelectric effect and impinge on the channel wall to multiply in number, eventually turning to a considerably amplified output.
G. W. Fraser states the MCP detecting the radiation of wavelength ranging from the X-ray region to the vacuum ultraviolet ray region and in particular, a quantum detecting efficiency of the MCP in the region of wavelengths from 0.6 to 600 .ANG. [Nuclear Instruments And Methods 195 (1982), p. 523-538]. A quantum detecting efficiency QE means a ratio of the number of discharged electrons to the number of incident photons and is represented by a function QE (.theta., E) of incident energy E [where E has the relation with a wavelength .lambda. that .lambda. (.ANG.)=12400/E (eV) and is equivalent] and an incident angle .theta. of the photon. FIG. 4A shows the relationship between the radiation of the wavelength .lambda. and the quantum detecting efficiency QE (10.degree., .lambda.) of the MCP. Further, FIG. 4B shows the relationship between the quantum detecting efficiency QE (8.degree., .lambda.), improved by combining the MCP with a CsI photocathode, and the wavelength .ANG. [Appl. Opt./Vol. 21, No. 23/p. 4206 (1982)].
FIG. 5 shows the absorption spectrum of the photoresist (PMMA) (The Spectroscopical Society of Japan, The 21st Summer Seminar, p. 81).
Although the X-ray optical system is primarily designed so that both the reflecting optical system and the diffracting optical system accommodate the X rays of the wavelength region selected in particular, the reflecting and diffracting optical systems mentioned above have properties of making the radiation of wavelengths other than selected ones incident on image surfaces in such a manner that the reflecting optical system has a high reflectance in regard to the radiation of long wavelengths such as vacuum ultraviolet rays and the diffracting optical system diffracts the radiation of long wavelengths and is pervious to the radiation of short wavelengths. Further, the detector also possesses a property of responding mostly to the radiation of a considerably wide wavelength range as mentioned above. Hence, if the X-ray radiation source has the property of emitting white radiation, the light existing outside a desired wavelength region will also traverse the optical system to be detected by the detector and will be mixed as a noise.
Although, therefore, the noise has been eliminated in the past in such a way that only the radiation of the desired wavelength region is selected from the X-ray source by using a spectroscope, problems have arisen that the use of the spectroscope leads to large size and high cost of the optical instrument.